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Addressing families of the victims who gathered in the footprint of the World Trade Center for the televised ceremony, the president recalled the heroism of rescue workers who helped save thousands of people trapped in the rubble after al-Qaida hijackers crashed commercial passenger jets into the skyscrapers and the Pentagon outside Washington. Another hijacked plane crashed in Pennsylvania.

The United States is "a nation that stands tall and united and unafraid because no act of terror can match the strength or the character of our country," the president said. "At the great wall and bedrock that embrace us today, nothing can ever break us. Nothing can change who we are as Americans."

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Expressing admiration for those who gave their lives to help save others, Obama cited the example of Welles Crowther, a 24 year old who led groups out of the South Tower and kept returning for others until the building collapsed, killing.

"To all those who responded with such courage ... on behalf of Michelle and myself and the American people, it is an honor for us to join in your memories, to recall and to reflect, but above all to reaffirm the true spirit of nine-eleven — love compassion, sacrifice."

Crowther's mother, Allison, watched the event with one of the people her son have saved.

"Welles believed that we are all connected as one human family, that we are here to look out for and to care for one another," she said. "This is life's most precious meaning."

Inside the National September 11 Memorial Museum, the last World Trade Center column to be cut away from Ground Zero is covered in graffiti messages by those who cleared the site after the attack, Ground Zero, New York City, May 15, 2014. (Adam Phillips/V

The museum and memorial plaza, set to open to the public next week, has been years in the making. Its completion was troubled by construction delays and continuing disputes over design and whether unidentified remains of some victims of should be buried in the depths of the new structure or above ground.

Former New York Mayor Michael Bloomberg said the musuem will hold meaning for all visitors, calling it a sacred marker of U.S. history destined to take its place alongside the fields of Gettysburg, the waters of Pearl Harbor and the Vietnam Veterans' memorial.

"The museum is a place where you can come to understand 9/11 through the lives of those who were killed and the lives of those who rushed here to help," Bloomberg said. "A place we come .... to honor acts of courage and compassion that saved lives and lifted spirits, the outstretched hands that rushed forward that day and in the hard weeks and months that followed."

Almost entirely underground, the museum extends to the bedrock where the steel supporting the World Trade Center was anchored.

Massive pieces of distorted steel recovered from the destroyed towers are on display along with some 10,000 artifacts from the day of the attack. There are pictures of those killed, a handwritten plea for rescue and voice recordings of people from around the world recalling where they were when they heard about the attack, and what they experienced as they watched the horror unfold on live television.

When entering and leaving the museum, visitors can look up through a glass ceiling, One World Trade Center, which scheduled to open later this year, while "The Rise of al-Qaida," a brief documentary film, can be viewed the at end of the exhibit.

One block from Thursday's ceremony, protesters under the banner of "9/11 Parents & Families of 9/11 Firefighters" decried the interment of loved ones' remains seven stories below ground, in what they call the museum's basement.

“We're not opposed to the museum, per se, with all the artifacts, and the story it is going to tell of history, but we are opposed [to] these remains being interred there," said Eileen Walsh, who lost her son, 27-year-old firefighter Michael Brennan, in the attack. "We had asked for a memorial tomb on the plaza when we found out there was going to be a plaza with the water pools. We wanted some input.”

Jim McCaffry, whose brother perished on the 78th floor of the South Tower, says he will not set foot in the museum unless the remains are moved above ground.

"Where is it acceptable that bureaucrats and politicians have more of a say in the interment of the human remains, particularly of those that died after an event such as 9-11?" he said. "They have more say than family members that loved them the most and knew them. In what society is that acceptable? I don't know."

David Ware, who was about five years old when the towers were hit, toured the memorial Thursday as part of a high school trip.

“I was in K-5 at the time, and something like this is important because so many people my age don't really have the same sense as to what happened and the same feeling of hurt and pain," he said. "This is something where our nation was attacked and this isn't just an event we can cast aside and not think about.”